add Context.ResponseWriter.IsHijacked to report whether the underline conn is already hijacked and a lot of cleanup and minor ws stress test example improvements

Former-commit-id: 444d4f0718d5c6d7544834c5e44dafb872980238

context/response_writer.go

@@ -41,6 +41,9 @@ type ResponseWriter interface {
 	// Here is the place which we can make the last checks or do a cleanup.
 	EndResponse()
 
+	// IsHijacked reports whether this response writer's connection is hijacked.
+	IsHijacked() bool
+
 	// Writef formats according to a format specifier and writes to the response.
 	//
 	// Returns the number of bytes written and any write error encountered.
@@ -195,6 +198,15 @@ func (w *responseWriter) tryWriteHeader() {
 	}
 }
 
+// IsHijacked reports whether this response writer's connection is hijacked.
+func (w *responseWriter) IsHijacked() bool {
+	// Note:
+	// A zero-byte `ResponseWriter.Write` on a hijacked connection will
+	// return `http.ErrHijacked` without any other side effects.
+	_, err := w.ResponseWriter.Write(nil)
+	return err == http.ErrHijacked
+}
+
 // Write writes to the client
 // If WriteHeader has not yet been called, Write calls
 // WriteHeader(http.StatusOK) before writing the data. If the Header